In the past, various systems have been devised for clamping the opposite edges of a printed circuit board heat conductive plate to a chassis which acts as a heat sink to carrry away heat generated by printed circuit components on opposite sides of the conductive plate. In many cases, the edges of the heat conductive plate are held within slots in the chassis by camming arrangements, typical camming arrangements being shown, for example, in U.S. Pat. Nos. 3,865,183; 3,970,198; 4,120,021, and 4,157,583.
Methods for increasing the heat transfer efficiency in printed circuit boards are receiving greater consideration in the overall packaging design of electronic systems. One reason for this is that maximum levels of device integration are being reached. In this respect, further component density cannot be efficiently compensated for by an associated increase in power dissipation.
One method of compensating for the increased power consumption is to reduce the resistance to heat flow occurring at the contact interface between the printed circuit board and the chassis or cold walls which support the board. Solid conduction through the actual contact area consists of many small points of contact. These are generally just a small fraction of the total contact area. The area depends upon the surface roughness, material malleability, flatness of the surfaces in contact and the contact pressure. Generally, the smoother the surface, the greater the number of points of actual contact. Also, the number of contact points and the area of contact at each point is related to pressure. As the Meyer hardness (i.e., resistance to indentation) is reached, the material at the points of contact will flow plastically until the Meyer hardness equals the contact pressure. This mechanism causes an increase in the true contact area to occur. In addition to solid conduction through the actual contact area, there is also conduction through an interstitial fluid or filler present in the void spaces between the points of actual contact. At low contact pressures, the major mode of heat transfer is through the void spaces.
Needless to say, it is highly desirable to maximize the cross-sectional contact area between the printed circuit board and the chassis in order to create the least possible resistance to heat flow. One difficulty with prior art camming arrangements is that the cam exerts a force on the conductive plate of the printed circuit board in one direction only. That is, the efficiency of the clamp is dependent on forcing a surface of the conductive plate against a single surface of the chassis. Conduction through the other side of the conductive plate is greatly limited since the surfaces of the cams which engage the side of the plate are usually very small in conducting area.